Amyloid-like aggregation or fibrillization of alpha-synuclein (alpha-Syn) and the filamentous deposits in Lewy bodies are believed to be closely associated with several fatal neurodegenerative disorders, including Parkinson's disease and Alzheimer's disease. Here, we report the importance of a nine-residue peptide motif, (66)VGGAVVTGV(74), in the fibrillization and cytotoxicity of human alpha-Syn. Mutagenesis combined with thioflavin T fluorescence detection, atomic force microscopic imaging, and cytotoxicity assays reveal that deletion of this sequence completely eliminates alpha-Syn fibrillization and cell toxicity. However, deletion of the (71)VTGV(74) sequence decreases the fibrillization rate while the cytotoxicity remains unchanged. Incorporation of charged residues within this region slows aggregation and even impedes filament formation. In addition, substitution of Gly68 with Ala or C-terminal truncations of alpha-Syn accelerate the fibrillization processes. Circular dichroism studies suggest that beta-sheet formation is often concomitant with filament formation. Thus, this segment, namely, the GAV motif, is responsible for aggregation or fibrillization of alpha-Syn and perhaps other amyloidogenic proteins. The oligomers formed during fibrillogenesis might be associated with the cytotoxicities of various alpha-Syn species. This finding may provide further insight into the understanding of the molecular mechanism underlying the fibrillogenesis implicated in neurodegeneration as well as aid in drug design and development of transgenic models.